Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - 3.7 versus 3.9

Having read virtually every diff archive on the board, I noticed that there is a consensus that the 3.7 is not as strong as 3.9 diff. Any particular reason for this consensus. I am about to go from a 3.9 to 3.7 with a change in tyres to give me a 10% better economy as I have decided against the 5 speed gearbox option.
Mark Wilson

Not having read the archives this is the first I've heard of any strength difference in the diffs. If there is any it is negligible. 3.7 is used on some of the circuits here without any signs of weakness, so if it's good enough to handle that much power under those conditions it's more than adequate for road use.
M H Allen

at 60mph, with 22.75" diameter tires, the RPM difference will be a little under 200 rpm

3450 -> 3275

I wonder if you'll really get a 10% fuel economy gain from that.

Norm Kerr

On a 1500 its NOT about fuel efficiency per se, it's about keeping the revs down = longevity of the 1500 by keeping engine temps down.
Deborah Evans

Hi guys,

Why not go for the overdrive conversion. Had mine converted for over 20 years, soooo much better.

Even fitted a 3.9 axle as it was too relaxed at speed.

Done 146,000 miles on its original engine. Who says the 1500 is weak? lol

Had an oil cooler from the day I got the car though, sure that's helped.

Regards Steve.
SR Smith 1

I fitted a 3.5 diff to my 1970 midget, and upgraded the engine to compensate for performance. 3500rpm at 70mph.
Geoff Mears

Even with the 3.7 ratio diff, the 1500 still seems undergeared. I can't understand why an overdrive was not available as standard as per the Spitfire.


I have a 3.5 (3.55 actually I think) in my 1500 which has plenty of torque to compensate for the higher gearing, and gives about 58mph at 3000rpm, using a gps rather than the speedo for speed measurement.

It is still undergeared though and still find myself reaching for the non existent fifth gear.

What is a non damaging cruising speed for these engines? (As from the factory, no balancing or oil cooler fitted)
It sounds quite frantic at 3000rpm but maybe thats just me being overly mechanically sympathetic.
JB Anderson

Thanks for all your comments. Just to say that I did a serious calculation in various tyres sizes based on the original 145SR and the diff change and one can get close to 10% change. Regarding the heating, I installed an oil cooler 25+ years ago and a Kenlow fan and basically had no hot moments at all. That is except running low on oil, idiot, and my water pump packing up twice in the cars history.
What surprised me most when I considered the gearbox change was my wife's comment - "No don't do that it will change the originality"
Mark Wilson

"I have a 3.5 (3.55 actually I think) in my 1500 "

What car is that originally from then?
Deborah Evans

I think the 3.5 was an aftermarket gear set.
Trevor Jessie

I am not sure but I think they were originally fitted to a Riley 1.5 or Wolseley model in the 60s or 70s, but I could be completely wrong about that.

I saw it advertised about 11 years ago so bought it as I wanted to raise the gearing a bit and found the 3.7 didn't do it for me, as they say.

What do you suggest is a sensible cruising engine speed for a 1500, in your experience?
I don't like taking it above 3000 which is fine for pootling about here, but maybe I am being too cautious, to put it mildly!
JB Anderson

If you have the cooling sorted then a 1500 will happily cruise at 4000 - 4500 rpm all day long. I used to regularly do long motorway runs at 5000 rpm in my Spitfire.

TBH an oil cooler is ESSENTIAL on a 1500 as I have posted on this forum on innumerable occasions ESPECIALLY in a Midget with its hot engine bay.

AFAIK Riley and Wolseley 1500s were generally fitted with the 3.9, with some of the late ones having a 3.7.
Deborah Evans

"British Leyland Special Tuning' made available a 3.55:1 crown wheel and pinion set, specifically for the 1500 Midget. When Special tuning was wound up (with BT), Graham Paddy bought up the stock (in the late 70's). I bought a couple of sets of the 3.55 cw and pinions from Graham back then. I fitted one set to my Midget and, rather foolishly, sold my spare set 'for a song' shortly after. I, too, have heard that the identical cw & pinion (ratio 3.55) was used in the Riley 1500.
Andy Hock


My understanding as to why the 1500 was never offered with overdrive was that it would have meant altering the floor pressing and gearbox tunnel, something that cash strapped BL just couldn't afford to do.

Remember that Triumph ruled the roost at BL in the 70's, BL just wouldn't spend money on an MG that was at the end of its life.

SR Smith 1

This thread was discussed between 25/05/2011 and 27/05/2011

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.